[Python-ideas] proto-PEP: Fixing Non-constant Default Arguments

[Chris Rebert]

Added to PEP draft. Thanks.
- Chris Rebert

Roman Susi wrote:
> Chris Rebert wrote:
>> Roman Susi wrote:
>>
>>> Hello!
>>>
>>> I'd liked to say outright that this bad idea which complicates matters
> 
> [skip]
> 
>>> P.S. However, I may be wrong. In that case my syntax suggestion would
>>> be this:
>>>
>>> def foo(non_const or []):
>>>    ...
>>>
>>> where [] is executed at runtime BECAUSE at def time non_const is
>>> somehow True and that is enough to leave [] alone.
>>> I have not checked, but I believe it is backward compatible.
>>> Anyway, could you summarize both contr-argument and this syntax
>>> proposal in the PEP?
>>
>> I don't quite understand exactly how this would work and would like more
>> details on it, but once you've explained it, of course I'd be happy to
>> include it in the next draft.
> 
> Simple.
> 
> 
> def foo(non_const or []):
>     ...
> 
> is equivalent to
> 
> def foo(non_const=None):
>     if non_const is None:
>         none_const = []
>     ...
> 
> 
> And this will be as before:
> 
> def foo(non_const=[]):
>     ...
> 
> Also, I thing that programmers should not use subtle difference between
> None and other False values, so something like
> 
> def foo(non_const=None):
>     non_const = none_const or []
> 
> is also valid.
> 
> Another approach (if you want to pursue the feature) could be
> complication to name binding protocol.
> 
> a = []
> 
> will be as before, but default value assignment could trigger some extra
> method. So, you can explicitly regulate your instance reaction to
> default-value assignment:
> 
> class MyMutableList:
>     ...
>     def __default__(self, old_default):
>         return old_default.copy()
> 
> 
> 
> 
> Regards,
> Roman
> 
> 
>> - Chris Rebert
>>
> 
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